The present invention relates generally to managing the availability of data in a storage system. More particularly the present invention relates to a method, apparatus and computer program for managing the availability of data in an external storage system such that the data remains in a highly available state even if a failure occurs in the external storage system.
Storage virtualization is the software implementation that combines multiple network storage devices into what appears to be a single storage unit. Storage virtualization is used, for example, in a Storage Area Network (SAN) which is a high-speed sub-network of shared storage devices. The SAN makes tasks such as archiving, back-up, and recovery easier and faster. Examples of storage virtualization are provided by U.S. Pat. No. 6,721,841 and “TagmaStore Universal Storage Platform”, Hitachi Data Systems Corporation, December 2004
U.S. Pat. No. 6,721,841 discloses a kind of virtualization method in which Input/Output (I/O) subsystems A and B are connected via a communication link. When the I/O subsystem A receives a read request from a host, the request is converted so as to read data in the I/O subsystem B. The data read from I/O subsystem B by I/O subsystem A is then supplied to the host from I/O subsystem. Thus, from the host point of view the data is being retrieved from I/O subsystem A.
“TagmaStore Universal Storage Platform” discloses a storage virtualization function in which a plurality of storage systems connected to the TagmaStore Universal Storage Platform are presented as if they were just internal storage devices.
A disadvantage of conventional storage virtualization is that the storage system enabling virtualization (primary storage system) cannot manage hardware failures that may occur in external storage systems such as hard disk drives. Therefore, in conventional storage virtualization if the external storage systems are not as reliable as the primary storage system, then data in the external storage systems may be lost even though the primary storage system is very reliable.